Thanks Jim,
Thanks also for publishing that tutorial.
I originally just made the choke part of the feed line. There are 6
loops so there are actually 7 passes through the cores, but that last
pass required a judicious application of wire pulling lube<:-)
Yes, the two wires are for a wider SWR spread. If time and body permit,
I want to add a third wire to get three different lengths. The end
forms a lopsided triangle There are a number of pulleys at the 95 ft
level on the 45G. I use a 3/8" double braid poly rope looped at the
bottom of the tower.
I'm thinking about doing the same on the 160 half sloper with switched
in additional inductance at the feed point, but I'll probably skip the
last part.
73
Roger (K8RI)
On 8/24/2015 11:39 AM, Jim Brown wrote:
On Sun,8/23/2015 11:49 AM, Roger (K8RI) wrote:
The reason for chokes was "it was handy". Being lazy, I decided to
try the second choke where the coax reached the tower as there was
already a connector there. The only extra work that entailed was
removing the weatherproofing from the connectors. It worked and that
let me get away without adding more weight hanging off the center of
the dipole.
That's a good move. The second choke reduces the common mode current,
reducing the stress on both chokes.
With the 5 core, 6 turn choke
http://www.rogerhalstead.com/ham_files/AntennaFeed2.htm plus nearly
100 feet of LMR-400 already hanging off the center. That dipole
already has well over 100# of tension to minimize the sag
I've got about 90# tension on mine, in the form of a water jug filled
with dry sand. Looking at your photo -- this choke is optimized for
30-15M. The wide spacing of the turns minimizes capacitance, so the
resonance is too high for 75M. Below 20M, it's best to closely space
the fewer number of turns that can be achieved with big coax.
I am building a single, larger choke, but using the 1/4" coax with a
Teflon dielectric and a double shield of silver plated braid. The
cores are spaced 1/8th inch and epoxied into the Lexan spacers. I've
not settled on the # of cores or turns of coax. The spacers are
strips of 1/4" Lexan 3/4" wide by a length to be determined. The
slots are milled 1/8th inch deep with 1/8th in spacing. I did one
trial using Plexiglass spacers with 1/4" deep slots, spaced 1/4", but
they were too fragile. The Plexiglass strips were cut from scrap and
were free. The Lexan was cut from a relatively small piece. Still
they were less than $2.00
Going to the Teflon coax is a good move in your system, because it
allows the greater number of turns needed for an effective choke. I
have not measured chokes wound with this type of coax -- the
dielectric affects the capacitance between turns, which can shift the
resonance a bit. I suggest that you follow my cookbook for RG8X-size
coax.
I'll have to shoot some photos of the spacers and the construction.
Nice looking, Roger. You say 75M fan -- I'm assuming multiple wires to
increase the SWR bandwidth on 75M only. My long fans are 2-band, with
a single wire for 80 and a second for 40.
73, Jim K9YC
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73
Roger (K8RI)
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